We changed the partitions but now we have to fromat it (the partition NOT the disk).

sudo mkfs.vfat /dev/sdb1

That's it, unplug the disk and plug it again. Now you can unplug it and plug it again to add the files on the disk.

Download the OS

On https://www.raspberrypi.org/downloads/ you download the operatiing system. For NOOBS you will have a .zip file with the bootable file. You just unzip the files in the SD disk. Raspbian is an .img file that has to be written on the disk, https://etcher.io/ is an easy way to flash the image file on the disk.

Now you are ready to plug the SD disk on the raspberry's slot, plug the HDMI, mouse and keyboard and finally connect the power cable. Rasberry will automatically reboot.